5.4.1 Phase Diagrams (Iron-Carbon Equilibrium Diagram)
Basically, the welding operation rapidly heats a metal to a temperature higher than its melting point.
During the heating process, atoms absorb energy and expand. When the metal reaches the melting
point, it transforms into a liquid. When the heat source is removed, the process is reversed.
Solidification of the weld puddle (from liquid to solid state) produces the weld bead. Figure 5.4 shows
a simplified iron-carbon diagram. This diagram allows metallurgists to see how adding carbon changes
the response of the steel to temperature changes. Phase diagrams are sometimes called “equilibrium
phase diagrams”. These diagrams show what structures are most stable at a given composition and
temperature.
Phase diagrams are created by cooling the material very slowly and thereby allowing the most
preferred phases to form. During welding, cooling rates are much faster than the equilibrium diagram.
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